This invention relates to flame scanners, and more specifically, to a silicon carbide photodiode based flame scanner that is suitable for use for purposes of effecting therewith individual burner flame discrimination in multi-fuel boilers and furnaces.
Flame scanners are important instruments in the operation of the combustion systems of fossil fuel-fired steam generators. To this end, flame scanners are one of the primary inputs into the burner management system with which such fossil fuel-fired steam generators are normally suitably provided. The principal function of such flame scanners is to monitor the combustion process, which is occurring within the fossil fuel-fired steam generators, and to provide a signal, based on such monitoring thereby of the combustion process within the fossil fuel-fired steam generator, when a stable flame exists so as to thereby provide therethrough an indication that it is safe to continue feeding fossil fuel into the combustion chamber of the fossil fuel-fired steam generator in which the combustion process is taking place. In the event that the flame becomes unstable, or the flame is lost completely, the flame scanner is designed to be operative to provide a loss of flame signal to the burner management system whereby the burner management system in turn is designed to be operative to shut off the fossil fuel to the fossil fuel-fired steam generator before an unsafe operating condition develops within the fossil fuel-fired steam generator.
A requirement that exists with fossil fuel-fired steam generators that employ a wall-fired firing system is that each individual burner thereof must have a flame scanner cooperatively associated therewith that is capable of establishing the presence of a flame from the burner with which the flame scanner is cooperatively associated. In contrast to fossil fuel-fired steam generators that employ a wall-fired firing system, in fossil fuel-fired steam generators that employ a tangentially-fired firing system the conventional philosophy for monitoring the combustion process, which is occurring therewith in, has been to simply establish that the so-called xe2x80x9cfireballxe2x80x9d, which is generated as a consequence of the combustion therewith in of the fossil fuel and air, is stable. To this end, the burner management system with which the fossil fuel-fired steam generators that employ a tangentially-fired firing system are equipped commonly utilize a two of four logic philosophy. In accordance with such two of four logic philosophy, at each elevation of the tangentially-fired firing system two of the four flame scanners on that elevation must be detecting the presence of a flame in order for it to be safe to continue feeding fuel to that elevation of the fossil fuel-fired steam generator. However, changes are today occurring in the industry whereby the operators of fossil fuel-fired steam generators that employ tangentially-fired firing systems are more and more looking to avail themselves of the same logic philosophy, insofar as flame scanning is concerned, as that which is utilized with fossil fuel-fired steam generators that employ wall-fired firing systems, that is, to provide a flame scanning means to monitor the flame of each individual burner.
Heretodate, the existing forms of flame scanners have relied on essentially two approaches to detect the presence of flame. The first of these two approaches involves the use of a photodiode to measure the light intensity from the flame. Based on such measurements an electrical current output is produced, which is proportional to the intensity of light on the photodiode. Typically, silicon and gallium phosphide photodiodes are utilized for such purpose. Moreover, frequently optical filters are employed to make the flame scanners more sensitive to particular wavelengths of light that are emitted from specific fossil fuels or under specific firing conditions. The second of these two approaches involves the use of an ultraviolet tube, which operates to produce a pulsed electrical output whose pulse rate is proportional to the intensity of the ultraviolet. In accordance with the teachings of the prior art, ultraviolet tubes historically have been the approach of choice for monitoring gas flames since the emission from the gas flame is primarily in the ultraviolet with only minimal visible light emissions. The photodiode approach, on the other hand, is used for monitoring oil flames and coal flames due to the high emissions therefrom, which are in the visible light and near infrared.
Notwithstanding the use heretofore in the prior art of the aforedescribed two approaches, it has unfortunately proven, however, to be difficult in many situations to be able to distinguish the near field burner flame from the background flame generated from adjacent burners. Generally, the primary method that is utilized for purposes of effecting a separation of the background flame from the near flame is to focus not only on an examination of the measured intensity but to also focus on an examination of the so-called xe2x80x9cflicker frequencyxe2x80x9d of the signal from the photodiode, which is being measured. To this end, it has been found that near field flame will typically have a higher xe2x80x9cflicker frequencyxe2x80x9d than do the background flames. Effecting a discrimination between the near field flame and the background flames has proven to be particularly difficult when the near field flame is being produced as a consequence of the firing of gas, whereas the background flames are being produced as a consequence of the firing of other forms of fossil fuels, such as oil or coal. The latter fossil fuels, i.e., oil and coal, are highly luminous, which means that the light emissions in the visible and near infrared are many orders of magnitude higher than the ultraviolet emissions being produced from the near field gas flame.
Continuing, the interference produced by the aforementioned background flames has been found to be more pronounced when silicon photodiodes are utilized. Moreover, this has been found to be so by virtue of the fact that silicon photodiodes have proven to be more responsive in the visible and near infrared spectral regions and by virtue of the fact that silicon photodiodes have been shown to not have very good response in the ultraviolet range, unless filtered. Ultraviolet tubes are sensitive in the ultraviolet range and as such may be deemed to provide better results than do silicon photodiodes under certain circumstances. However, ultraviolet tubes also have historically been shown to possess some drawbacks. Namely, ultraviolet tubes have relatively limited operational lives and mechanical shutters are typically required to provide some feedback that the tubes are operating correctly. Additionally, ultraviolet tubes require complex circuitry to permit extraction and differentiation of both intensity and flicker frequency signals.
To thus summarize, the problem with which operators of fossil fuel-fired steam generators are being faced is their need to have a flame scanner that is reliable and yet is capable of distinguishing near field flame operation over background flames. Moreover, such a flame scanner must be capable of doing so under all firing conditions and without requiring that the setpoints be changed on such a flame scanner when the type of fossil fuel being fired in the fossil fuel-fired steam generator changes.
There have been numerous modifications made over the years insofar as flame scanners that are suitable for use for purposes of effecting therewith the detection of the presence of a flame in fossil fuel-fired steam generators are concerned. By way of exemplification and not limitation in this regard, the resultant of one such modification is the flame scanner that forms the subject matter of U.S. Pat. No. 3,185,846 entitled xe2x80x9cUltra-Violet Radiation Flame Monitorxe2x80x9d, which issued on May 25, 1965. In accordance with the teachings of U.S. Pat. No. 3,185,846, a device is provided for monitoring the flame in a combustion chamber, the flame being produced as a consequence of the burning within the combustion chamber of a fuel containing hydrogen and carbon. The subject device comprises a casing adapted to be mounted on the wall of the combustion chamber, a tubular extension extending from said casing into the combustion chamber, and a radiation responsive detector mounted in said extension adjacent the end nearest the flame to receive radiation therefrom, and an electric circuit positioned within said casing responsive to conduction or non-conduction of said detector to manifest the existence or non-existence of the flame. Continuing, the radiation responsive detector is sensitive only to invisible ultraviolet radiation produced by the burning of hydrogen in the fuel and operative to become electrically conductive in response to ultraviolet radiation.
The resultant of a further one of such modifications is the flame scanner that forms the subject matter of U.S. Pat. No. 3,280,882 entitled xe2x80x9cFlame detector Arrangementxe2x80x9d, which issued on Oct. 25, 1966. In accordance with the teachings of U.S. Pat. No. 3,280,882, there is provided a flame detector, which is designed to be so positioned relative to the burner it is supervising as to positively discriminate between the monitored burner and adjacent burners and to provide a sensitivity sufficient not to cause false interruptions of burner fuel supply due to the normal variations in burner operation. To this end, in accordance with the teachings of U.S. Pat. No. 3,280,882 an ultraviolet flame sensing device is associated with a fuel burner assembly including a tubular burner nozzle adapted to pass a stream of pulverized coal and carrier air through a port formed in a boundary wall of a furnace, with provisions for mixing additional combustion air with and dispersing the fuel as it passes through the burner port. The aforesaid fuel dispersal provisions include an impeller normally located outside of and close to the discharge end of the burner nozzle and supported by and attached to one end of a pipe extending coaxially through the burner nozzle. The flame sensing device comprises a detector tube disposed within the impeller support pipe and contiguous to the impeller and looking directly into the bright ignition zone immediately downstream of the impeller. This detector tube location provides a clear and unobstructed view of the ignition zone and a flame signal of optimum reliability in respect to sensitivity and discrimination.
The resultant of yet a further one of such modifications is the flame scanner that forms the subject matter of U.S. Pat. No. 3,321,634 entitled xe2x80x9cPhotosensitive Flame Monitoring Circuitxe2x80x9d, which issued on May 23, 1967. In accordance with the teachings of U.S. Pat. No. 3,321,634 there is provided a circuit for flame monitoring apparatus, which is adapted to provide a signal representative of the ratio of flame fluctuations to average flame brightness including a photo-conductive cell, a silicon junction diode or device of equivalent character connected in series with the cell and arranged substantially to compensate by its increased A. C. resistance at lower currents for diminished flicker response of the cell at lower illuminations, and means for passing an A. C. signal representative of radiation fluctuations at the cell to a second silicon junction diode or device of equivalent character receiving a current controlled by the cell and representative of average illumination of the cell and providing an A. C. output signal.
The resultant of yet still a further one of such modifications is the flame scanner that forms the subject matter of U.S. Pat. No. 4,051,375 entitled xe2x80x9cDiscriminating Flame Detectorxe2x80x9d, which issued on Sep. 27, 1977 to the same assignee as that to which the present patent application is assigned. In accordance with the teachings of U.S. Pat. No. 4,051,375, a device is provided for discriminating between the presence of flame in a selected position and the presence of flame in nearby locations within a combustion area. A means for detecting electromagnetic radiation having wavelengths known to be characteristic of the base of the flame but not of the rest of the flame is aimed at the position at which the base of the flame to be monitored will occur if the flame is present. The intensity of the radiation having the given wavelengths is compared with the intensity of radiation having nearby wavelengths, and the relative magnitudes of the two intensities are used as an indication of whether flame is present or absent at the selected position.
The resultant of another such modification is the flame scanner that forms the subject matter of U.S. Pat. No. 4,368,031 entitled xe2x80x9cStationary Flame Scanner For Tilting Burnerxe2x80x9d, which issued on Jan. 11, 1983 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,368,031, a flame monitoring system is provided for use on the furnace of a vapor generator employing tiltable burners, and particularly on a furnace equipped with tilting burners arranged in accord with the tangential firing method.
Each burner is provided with its own individual scanner. Each scanner is disposed to sight transversely across the base of the flame emanating from its associated burner. A plurality of at least three light transmission tubes are stationarily mounted within each scanner head with at least one tube mounted to sight at an upward acute angle across the flame, at least one other tube mounted to sight horizontally across the flame, and at least a third tube mounted to sight at a downward acute angle across the flame. A separate fireball scanner is mounted in the waterwall and aimed to sight at the center of the furnace to monitor the fireball formed therein by the flames emanating from the individual burners.
The resultant of yet another such modification is the flame scanner that forms the subject matter of U.S. Pat. No. 5,073,104 entitled xe2x80x9cFlame Detectionxe2x80x9d, which issued on Dec. 17, 1991. In accordance with the teachings of U.S. Pat. No. 5,073,104, there is provided a method and apparatus of detecting the condition of a flame having an emf by electrically conducting the emf generated by the flame as a signal to a sensor through an electrically isolated conductor means and sensing with said sensor an electrical parameter, which is a measure of the emf of the flame, and wherein the parameter is the ratio of the A.C. and D.C. signal levels.
Thus, although the numerous modifications, which have been made over the years especially of the type to which reference, by way of exemplification and not limitation, has been had hereinbefore insofar as flame scanners that are suitable for use for purposes of effecting therewith the detection of the presence of a flame in fossil fuel-fired steam generators are concerned, allegedly have been demonstrated to be operative for the purpose for which they have been intended, there has nevertheless been evidenced in the prior art a need for a new and improved flame scanner that would be suitable for use particularly for purposes of effecting therewith individual burner flame discrimination in multi-fuel fired steam generators, i.e., boilers and furnaces. More specifically, a need has been evidenced in the prior art for such a new and improved flame scanner that is characterized, by way of exemplification and not limitation, in that the flame scanner while being reliable nevertheless possesses the capability of being able to distinguish near flame operation over background flame. Moreover, such a new and improved flame scanner must possess such a capability of being able to distinguish near field flame operation over background flame under all firing conditions, and also in addition must be able to do so without having to necessitate that the setpoints on such a new and improved flame scanner be changed when there is a change in fuel type.
It is an object of the present invention to provide a new and improved flame scanner particularly suitable for employment in steam generators, i.e., boilers and furnaces.
It is a further object of the present invention to provide such a new and improved flame scanner, which is characterized in that when so employed in steam generators is operative both for purposes of reliably monitoring an individual burner flame and for purposes of detecting, based on such monitoring, unstable operation or flame-out conditions over all background firing conditions.
It is another object of the present invention to provide such a new and improved flame scanner, which is characterized in that the operation thereof is based on the use of a solid state photodiode, which is operative for purposes of effecting therewith the conversion of ultraviolet (UV) energy from the individual burner flame into electrical signals.
Another object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that the operation thereof is that of a multi-fuel flame scanner, which is designed for use with steam generators, i.e., boilers and furnaces, that fire gas, oil, coal, or any combination of these three fuels.
And still another object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that years of reliable service in a power plant environment are capable of being derived therefrom.
A further object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that the sensitivity thereof is only to the ultraviolet (UV) energy that is emitted from a near field burner flame.
Yet an object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that a near field flame signal is produced thereby, which has clear, distinct features over the signal produced by background radiation alone.
Yet a further object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that the presence of burner flame can be reliably determined thereby without having to change trip setpoints when fuels are changed.
Yet another object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized in that the use thereof is equally well suited either for new applications or for retrofit applications.
Yet still another object of the present invention is to provide such a new and improved solid state photodiode based flame scanner, which is characterized by being compact, robust and easy to install.
In accordance with the present invention there is provided a new and improved silicon carbide photodiode based flame scanner that is suitable for use for purposes of effecting therewith individual burner flame discrimination in multi-fuel fired steam generators, i.e., boilers and furnaces.
The subject silicon carbide photodiode based flame scanner is predicated on the use of a specific solid state photodiode that is only sensitive to the ultraviolet (UV) energy emitted from a near field burner flame. To this end, the high flame sensitivity achieved thereby by monitoring in the ultraviolet (UV) range is combined with the high reliability, stability and sensitivity associated with solid state photodiodes. The high sensitivity to flame ultraviolet (UV) emissions made possible through the use of the specific solid state photodiode, i.e., the silicon carbide photodiode, allows the subject silicon carbide photodiode based flame scanner to produce a near field flame signal that has clear, distinct features over the signal produced by background radiation alone. As such, this means that the subject silicon carbide photodiode based flame scanner can readily determine the presence of burner flame without having to change trip setpoints when fuels are changed.
In accordance with the present invention, the specific new silicon carbide photodiode upon which the subject silicon carbide photodiode based flame scanner is predicated replaces the conventional photodiodes heretofore employed in the prior art. More specifically, the specific new silicon carbide photodiode upon which in accordance with the present invention the subject silicon carbide photodiode based flame scanner is predicated has a spectral response curve wherein this specific new silicon carbide photodiode is very responsive in the ultraviolet (UV) range, i.e., up to 380 manometers, with little to no response in the visible or higher range, i.e., greater than 380 nanometers. Moreover, the response range thereof is spectrally broader than ultraviolet (UV) tubes and envelopes the significant ultraviolet (UV) emission bands that ultraviolet (UV) tubes miss. Further, the upper, i.e., longer wavelength, range end of the response curve of the specific new silicon carbide photodiode based flame scanner upon which in accordance with the present invention the subject silicon carbide photodiode based flame scanner is predicated overlaps the emission continuum that is characteristic of oil and coal flames. However, the responsivity of the specific new silicon carbide photodiode naturally falls from 30% to 10% in this wavelength region. The result is that the signal amplitude sensitivity of the specific new silicon carbide photodiode is of similar magnitude to either ultraviolet (UV) emissions or the oil/coal emission continuum. This permits the specific new silicon carbide photodiode to be more responsive to the near field flame versus any highly luminescent background flames of the visible and NIR spectrum.
The specific new silicon carbide photodiode upon which in accordance with the present invention the subject silicon carbide photodiode based flame scanner is predicated is advantageously characterized over the ultraviolet (UV) tube in that the output signal of the specific new silicon carbide photodiode provides a continuous analog signal versus the pulsed output signal, which is supplied by the ultraviolet (UV) tube. Moreover, the electronics needed to support the high voltage supply for ultraviolet (UV) tubes are also not required with the specific new silicon carbide photodiode, nor is the mechanical shutter mechanism required therewith. Furthermore, the specific new silicon carbide photodiode upon which in accordance with the present invention the subject silicon carbide photodiode based flame scanner is predicated is advantageously characterized over conventional other forms of solid state photodiodes, such as silicon (Si) and gallium phosphide (GaP) photodiodes, in that the specific new silicon carbide photodiode has much greater responsivity in the ultraviolet (UV) range and also in that additional optical filtering is not required therewith.